A knowledge base (KB) is a good example of a large database that stores binary relation tuples about real world entities. Freebase [1]}, Wikidata [2]}, and DBpedia [3]} are examples of large KBs. Figure REF  gives some idea about the structure of a KB. This example KB has five entities: Barack Obama and Michele Obama of PERSON type, and United States of America, Hawaii, and Honolulu of LOCATION type. In addition, there exist a few relations among them, such as spouse, lives_in, located_in, and capital.
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